The invention relates to the field of quadrupole mass analyzers, and more specifically to a method of linearizing ion currents in a quadrupole gas analyzer.
Mass spectrometers include instruments which ionize a gas sample, separate a resulting beam of ions by mass to charge ratio, and then detect filtered ions as an electrical signal. The masses, unique for each substance, identifies the gas molecules from which the ions were created. One such mass filter used is referred to a quadrupole which consists of four parallel electrodes or poles arranged in a square array. Opposite poles are connected together electrically such that an electric field of hyperbolic geometry is produced. Potentials applied to these poles are a superposition of variable DC and RF voltages, generally of a fixed RF frequency.
The above instrument works preferably in a high vacuum environment because the ions, once created, must not collide with other gas molecules as the molecules move through the instrument; otherwise some molecules may not be detected. High vacuum means pressures below 1.3E-2 Pascals, or greater, or approximately 1-0E-4 torr.
It has been determined that the transmission of ions in a quadrupole mass spectrometer suffers losses in the high operating pressure area due to several effects; most notably from collisions between ions and neutral gas molecules, ion scattering and coulombic repulsion. The above effects results in a perceived non-linearity between pressure and ion current. It would be desirable to provide a sensor which could correct such non-linear deficiencies, or alternately to provide a technique for correcting measured ion current data to expand the performance of a quadrupole mass analyzer in a higher pressure regime than currently available.
A primary object of the present invention is to extend the linearization of an ion detector in a mass quadrupole sensor system over higher pressures than similar analyzers of the prior art.
Another primary object of the present invention is to be able to correct ion current data over an increased range of gas pressures to take into account the effects of ions at higher pressures.
Therefore, and according to a preferred aspect of the present invention, there is provided a method for correcting a measurement of a partial pressure in a mass spectrometer measuring both a measured partial pressure and a measured total pressure in the millitorr range of a high vacuum, the method comprising the steps of:
measuring a total pressure ion current to obtain a measured total pressure ion current;
determining, based on said measured total pressure ion current, said measured total pressure;
measuring a partial pressure ion current to obtain a measured partial pressure ion current;
determining a correction factor based on said measured total pressure ion current; and
correcting, using said correction factor, said measured partial pressure to obtain a corrected partial pressure.
Preferably, correction factors can be applied to extend the pressure range for linear partial pressure measurement from the mass analyzed ion current by scaling the measured ion current with a pressure dependent or an ion intensity dependent scaling factor.
More preferably, the pressure dependent correction factor may have a known functional dependence due to scattering of ions out of the detected beam over the path length of the ion, a loss determinable by the relation:
Icorrected=Im+*ebP
in which Im+ is the measured ion current corrected by an empirically determined constant, b and P is the total pressure.
The pressure dependent factor may have another component due to non-gas scattering while another correction factor dependent on ion density can also be determined from measured deviations from linear response. Each of the correction factors can be empirically or mathematically determined dependent on the measured ion current, the total gas pressure and/or the ion density.
An advantage of the present invention is that a linearization factor can be applied to extend the pressure range of a gas analyzer.
Another advantage of the present invention is that each correction factor can be directly applied using software contained in a suitable gas analysis system, or alternately applied using interpolation tables or calculated and applied directly to empirical data.
Other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.